[Firecracker]

Three reasons often given:

(1) Gives an incentive to deploy money. "Use it or lose it." This stimulates investment and consumption.

(2) Some prices are "sticky down," like wages; people respond much better to nominal wage increases than pay cuts. Inflation lets these prices be adjusted down more easily if needs be.

(3) Stealth tax, not only on the holders of dollar-denominated wealth but also because inflation (a) pushes people into higher tax brackets and (b) creates more taxable "capital gains". If you're a politician, it's nice to be able to pass repeated tax cuts, even as government expenditures continue to grow as a fraction of GDP.

All that said, relatively stable price levels are important. Without them, contracts get tricky, particularly around inventory or agreements into the future.